Phosphoramide additives for preparation of organic carbonates

ABSTRACT

An organic carbonate such as dimethyl carbonate is prepared by reacting an alcohol such as methanol with carbon monoxide oxygen in the presence of a catalyst system containing Cu(OMe)Cl as a catalyst and a phosphoramide as an additive for the catalyst system.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the preparation of organic carbonates. More particularly it relates to additives or cosolvents for the preparation of dimethyl carbonate (DMC). DMC may be used as a gasoline extender and octane enhancer, as an organic solvent, or as a reactant in place of phosgene in the preparation of isocyanates, polycarbonates, and various agricultural and pharmaceutical intermediates.

Dimethyl carbonate (DMC) may be prepared by the reaction of methanol with carbon monoxide and oxygen in the presence of a catalyst system. Those skilled in the art, constantly seek new or improved catalysts or cosolvents for such catalysts to lower costs to improve reaction conditions, the yield and rate of production, and to facilitate separation of reactants, products, and catalysts.

Thus, it is an object of this invention to provide an improved method of preparing dimethyl carbonate, lower the cost of the process, substantially increase the yield and rate of production of dimethyl carbonate, and ease the separation of DMC and water from the catalyst system.

2. Disclosure Statement

U.S. Pat. No. 3,114,762 discloses as catalysts metal salts including chlorides and bromides of platinum and palladium plus an oxidizing agent such as iron or copper salts having the same anion.

U.S. Pat. No. 3,227,740 discloses as catalyst mercuric halides or carboxylates.

Saegusa et al, J. Org. Chem., 35, 2976-2978 (1970) discloses the reaction of CO with copper alkoxides including the dimethoxide, the di-allyloxide, the chloride methoxide, and the acetylacetonate methoxide.

Romano et al IEC Prod. Res. Dev. 19, 396-403 (1980) discloses as catalyst cuprous chloride/cupric chloride methoxide.

U.S. Pat. No. 4,218,391 discloses as catalysts salts of metals of Group IB, IIB, and VIII, preferably monovalent copper such as cuprous bromide, chloride, or perchlorate.

U.S. Pat. No. 4,318,862 discloses as catalyst salts of metals of Groups IB, IIB, or VIII, typically a copper salt such as CuCl.

U.S. Pat. No. 3,846,468 discloses as catalysts cuprous chloride complexes with an organic ligand such as pyridine, dipyridyl, imidazole, phenanthroline, alkyl, or aryl phosphines, dimethyl sulfoxide, dimethyl formamide, quinuclidine, acetonitrile, benzonitrile, malonitrile, succinodinitrile, or adiponitrile.

U.S. Pat. No. 3,980,690 discloses as catalyst a complex of copper chloride and poly-4-vinylpyridine.

Rivetti et al, J. Organometallic Chem, 174 (1979) 221-226 discloses as catalysts palladium (II) complexes in the presence of ligands and added bases. Alkyl phosphines are said to inhibit carbonylation almost completely. The presence of tertiary amines enhances the formation of dimethyl carbonate. Low yields (6% or less) of dimethyl carbonate are obtained with Pd(OAc)₂ in the presence of ligands such as R₃ P where R is p--C₆ H₄ OCH₃. Yield is increased to 61% in the presence of a base such as diisopropylethylamine.

U.S. Pat. No. 3,952,045 discloses as catalysts organic phosphorus compounds such as phosphine oxide, phosphite, phosphate, or phosphonate plus copper halides.

U.S. Pat. No. 4,360,477 discloses a catalysts cupric halides inter alia.

Yang et al CA 86, 171868u (1977) discloses as catalysts PdCl₂, CuCl₂, MnCl₂, and LiCl.

Lapidus et al CA 93, 72338j (1980) discloses as catalyst MnCl₂, KMnO₄, CuCl₂, LiCl, and Mn(OAc)₃.

Itatani, Japanese patent publication No. 54-24827 pub Feb. 24, 1979 discloses as catalyst a cuprous halide plus as auxiliary catalyst a halide of an alkali metal or an alkaline earth metal.

U.S. Pat. No. 4,370,275 discloses as catalyst compositions containing copper, chemically bonded oxygen, and halogen and a nitrogen base. A typical catalyst contains CuO or Cu(OCl)₂ and n-butylamine inter alia. Preferred combinations include: CuCO₃, Cu(OH)₂ ; CuCl₂ and pyridine hydrochloride etc.

U.S. Pat. No. 4,131,521 discloses an electrochemical process utilizing a non-fluoride halide-containing electrolyte.

U.S. Pat. No. 4,113,762 discloses as catalyst a complex of copper (as CuCl) with VCl₃, CrCl₃ FeCl₃, CoCl₂ AlCl₃, or SiCl₄.

U.S. Pat. No. 4,361,519 discloses as catalyst (i) a Bronsted base such as a quaternary ammonium, phosphonium, or sulfonium compound or an alkoxide or hydroxide of alkali metal or alkaline earth metal or a salt of a strong base and a weak acid or amines etc. plus (ii) a Group VIII B element Ru, Rh, Pd, Os, Ir or Pt plus (iii) oxygen plus (IV) a redox catalyst such as a Mn or Co containing catalyst. A typical system includes (i) a pentamethylpiperidine, (ii) PdBr₂ and (iii) pyridine adduct of salicylaldehyde-ethylene diamine C (II) complex.

European Pat. No. 0,071,286 discloses as catalyst a copper compound such as a halide (in the presence of an amine) plus a sulphone such as dimethyl sulphone or sulfolane.

SUMMARY OF THE INVENTION

This invention is directed to a method of preparing an organic carbonate R₂ CO₃ wherein R is a hydrocarbon group selected from the group consisting of alkyl, alkaryl, aralkyl, cycloalkyl, and aryl hydrocarbon groups. The method comprises reacting an alcohol with carbon monoxide and oxygen in the presence of a catalyst system containing

(a) as a catalyst, a copper hydrocarbonoxy halide Cu(OR')X wherein R' is a hydrocarbon group selected from the group consisting of alkyl, alkaryl, aralkyl, cycloalkyl, and aryl and X is a halide; and

(b) as an additive for the catalyst, a phosphoramide of the formula ##STR1## where R₁ and R₂ are (C₁ -C₈) alkyl, alkylene or oxa-, thia-, or aza-alkylene groups; and recovering the organic carbonate product.

DESCRIPTION OF THE INVENTION

In preparing organic carbonates according to the present invention, a phosphoramide additive, i.e., cosolvent for the catalyst is used which substantially increases the rate of production of the organic carbonate. An organic carbonate such as dimethyl carbonate is prepared by the oxidative carbonylation of alcohols in the presence of a catalyst and a phosphoramide additive, i.e., cosolvent of the present invention.

The organic carbonate, e.g., dimethyl carbonate, is prepared by reacting an alcohol with carbon monoxide and oxygen in the presence of a catalyst system containing

(a) as a catalyst, a copper hydrocarbonoxy halide Cu(OR')X wherein R' is a hydrocarbon group selected from the group consisting of alkyl, alkaryl, aralkyl, cycloalkyl, and aryl and X is a halide; and

(b) as an additive for the catalyst, a phosphoramide of the formula ##STR2## where R₁ and R₂ are (C₁ -C₈) alkyl, alkylene or oxa-, thia-, or aza-alkylene groups; and recovering the organic carbonate product.

The charge alcohol which may be employed in practice of the method of this invention may include those characterized by the formula ROH.

In the above compound, R may be a hydrocarbon group selected from the group consisting of alkyl, aralkyl, cycloalkyl, aryl, and alkaryl, including such radicals when inertly substituted. When R is alkyl, it may typically be methyl, ethyl, n-propyl, iso-propyl, n-butyl, i-butyl, sec-butyl, amyl, octyl, decyl, octadecyl, etc. When R is aralkyl, it may typically be benzyl, beta-phenylethyl, etc. When R is cycloalkyl, it may typically be cyclohexyl, cycloheptyl, cyclooctyl, 2-methylcycloheptyl, 3-butylcyclohexyl, 3-methylcyclohexyl, etc. When R is aryl, it may typically be phenyl, naphthyl, etc. When R is alkaryl, it may typically be tolyl, xylyl, etc. R may be inertly substituted i.e. it may bear a non-reactive substituent such as alkyl, aryl, cycloalkyl, ether, etc. Typically inertly substituted R groups may include 2-ethoxyethyl, carboethoxymethyl, 4-methyl cyclohexyl, etc. The preferred R groups may be lower alkyl, i.e. C₁ -C₁₀ alkyl, groups including methyl, ethyl, n-propyl, i-propyl, butyls, amyls, hexyls, octyls, decyls, etc. R may preferably be methyl.

The charge alcohol may be a phenol i.e. when R is aryl. The notation ROH is intended to include polyols such as ethylene glycol, glycerine, sorbitol, poly(oxyalkylene) polyols, etc; in these latter compounds, the formula may more typically be represented as R(OH)_(n) wherein R is derived from an alkyl group and n is an integer, typically 2-10.

The charge alcohols which may be employed include these listed below in Table I.

                  TABLE I     ______________________________________             methanol             ethanol             n-propanol             i-propanol             benzyl alcohol             phenol             ethylene glycol             glycerine             sorbitol             poly(oxyethylene-10) glycol     ______________________________________

The preferred alcohols are the lower (C₁ -C₄) alkanols; and most preferred is methanol.

The carbon monoxide charge which may be employed may be a pure gas. More commonly it may be a synthesis gas of high purity from which most of the hydrogen and carbon dioxide have been removed.

The catalyst, i.e., copper hydrocarbonoxy halide Cu(OR')X may be one wherein X is fluorine, chlorine, bromine, or iodine. Preferably X is chlorine or bromine and more preferably chlorine.

R' may be selected from the same group as R; and preferably R' is lower, alkyl i.e. C₁ -C₁₀ alkyl. Preferably R' is methyl. Typical compounds may include those listed below in Table II. The preferred catalyst being the first listed, i.e., cupric methoxychloride.

                  TABLE II     ______________________________________                  ##STR3##                  ##STR4##                  ##STR5##     ______________________________________

The copper salt catalyst system containing Cu(II)(OMe)Cl/Cu(I)Cl is only sparingly soluble in methanol and the rate of DMC formation is undesirably low. This invention relates to the large increase in the yield and rate of organic carbonate formation in the presence of a phosphoramide additive or cosolvent. Thus, the rate of DMC production is significantly augmented in the presence of hexamethylphosphoramide (HMPA) as a cosolvent for the catalyst concerned. According to the present invention, it has been found that the rate of DMC formation in HMPA is several times higher than that under similar conditions in either methanol or dimethyl phthalate. An additional advantage of HMPA (or similar amide cosolvents) over various lower boiling point solvents (such as lower hydrocarbons, ethers, and acetonitrile) is that HMPA (bp 230° C.) is less volatile and remains with the catalyst during any subsequent separation of the reaction mixture by any flash or distillation method.

The additive for the catalyst may be a phosphoramide of the formula ##STR6## where R₁ and R₂ are (C₁ -C₈) alkyl, alkylene or oxa-, thia-, or aza-alkylene groups.

The phosphoramides, according to the present invention may be hexamethylphosphoramide, hexaethylphosphoramide, tri(pentamethylene)phorphoramide or tri(oxapentamethylene)phorphoramide.

Also, according to formula (I), the phosphoramides (e.g., cosolvents) may be represented by the following formulas:

Hexamethylephophoramide (where R₁ and R₂ are each CH₃) ##STR7##

Hexaethylphosphoramide (where R₁ and R₂ are each C₂ H₅) ##STR8##

Tri(pentamethylene)phosphoramide (where R₁ and R₂ combined are pentamethylene) ##STR9##

Tri(oxapentamethylene)phosphoramide (where R₁ and R₂ combined are oxapentamethylene) ##STR10##

The catalyst system may be present in the reaction mixture in an amount of about 0.01 to about 50 parts, preferably about 0.1 to about 20 parts, and more preferably about 10 parts per 100 parts of charge methanol.

The practice of the method of this invention may be carried out by adding 100 parts of an optimum mixture of the alcohol ROH, such as methanol and an amide cosolvent to the reaction mixture. The weight percent (wt.%) of cosolvent in methanol may vary from 1 to 95%, preferably about 50%, and more preferably about 75%. The catalyst system may then be added. The system is then subjected to inert gas typically nitrogen at a partial pressure of 5-1000 psi, preferably 100-300 psi, say about 100 psi and heated to 20° C.-170° C., preferably 80°-120° C., say about 90° C. at a total pressure of 10-2000 psi, preferably 150-600 psi, say about 150 psi over 0.25-2 hours, say about 0.5 hour.

Carbon monoxide-containing gas is then admitted to a carbon monoxide partial pressure of 5-3000 psi, preferably 100-900 psi, say 350 psi over 0.25-10 hours, say 1 hour.

During this period, the following reaction occurs in the preferred embodiments:

    2Cu(OMe)Cl+CO→(MeO).sub.2 CO+2CuCl.

At the end of this time, the reaction mixture may be rapidly cooled to 20° C.-90° C., say 25° C. at a total pressure of 15-3000 psi, say 350 psi.

The reaction mixture may be flashed under reduced pressure to remove gases, methanol, DMC, and water or depressured and then distilled to azeotropically distill off various fractions containing methanol, dimethyl carbonate and water. The product may be further treated to effect greater purification of the impure dimethyl carbonate.

The residual catalyst system (0.1-50 parts, say 10 parts) may be regenerated as by bubbling oxygen-containing gas, typically air at 20° C.-65° C., say 45° C. for 1-50 hours, say 6 hours in the presence of an alcohol cosolvent combination, typically methanol HMPA (3 to 1) in an amount of 100 parts.

During this regeneration step, the following reaction occurs in the preferred embodiment:

    2CuCl+2MeOH+1/2O.sub.2 →2Cu(OMe)Cl+H.sub.2 O

At the end of the regeneration period, the catalyst in methanol may be recycled if the water content is less than about 1.0 wt.%. If more water than this is present, the catalyst and the cosolvent may be dried by depressurizing and flashing all volatiles including DMC and water and the anhydrous catalyst system may be recycled using anhydrous methanol.

Practice of the method of this invention will be apparent to those skilled in the art from the following examples, wherein as elsewhere in this specification, all parts are parts by weight unless otherwise noted.

EXAMPLES I-IV

In these examples which represent the best mode known of practicing the method of this invention, there was added to the reaction vessel 141 ml of anhydrous methanol and 18.2 g (0.14 mol) of anhydrous Cu(OMe)Cl and 47 ml of cosolvent (i.e., aectonitrile, triglyme, dimethyl phthalate or hexamethylphorphoramide). The methanol and phorphoramide cosolvent were added in a ratio of methanol to cosolvent of about 3:1.

In each of the examples, the procedure and conditions were the same except that the cosolvent was different.

The reaction mixture was pressurized to 100 psig with nitrogen, heated to 90° C. and maintained at 90° C. for 0.5 hour. The pressure was increased to 500 psig with carbon monoxide and the stirring was continued for 0.25 hour. The reaction mixture was then cooled to room temperature, and depressurized. The reaction mixture was distilled with added methanol (200 ml) to recover azeotrope containing methanol and dimethyl carbonate. Analysis by Gas Chromatography for each run indicated the yields (based on copper salt) shown in Table III.

Methanol (100 ml) was added and the catalyst was regenerated by bubbling air through the suspension at 45° C. for 6 hours. The regenerated catalyst in this reaction mixture was recycled for reaction with CO under standard reaction conditions as described above. The product was recovered and analyzed for DMC as mentioned above. The numbers in parentheses represent the yield attained in a subsequent run in which the catalyst used has been regenerated, the reaction conditions being otherwise the same.

                  TABLE III     ______________________________________     Production of DMC     Example            Cosolvent   B.P. (%) of Cosolvent                                        Yield (%).sup.1     ______________________________________     R.sup.2            Methanol (neat)                         65              8(14)     I      Acetonitrile                         81             4(5)     IV     Triglyme    216             6(9)     III    Dimethyl    282             9(8)            Phthalate     IV     Hexamethyl  230             48(64)            Phosphoric            Triamide     ______________________________________      .sup.1 Gas chromatographic analysis using a Porapak P (100-120 mesh)      column was employed to determine the yield based on copper salt added and      100% DMC product selectivity.      .sup.2 Reference cosolvent which was compared with present cosolvents.

As shown in Table III above, the addition of a cosolvent of the present invention greatly increased the yield of dimethyl carbonate (DMC). In fact, the yield was more than 4 to 6 times the yield when methanol was used. It is apparent that the process of this invention makes it possible to obtain higher yields of DMC in shorter time, i.e., to increase the rate of formation of desired. DMC.

Although this invention has been illustrated by reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made which already fall within the scope of this invention. 

I claim:
 1. A method of preparing dimethyl carbonate which comprises reacting methanol (CH₃ OH) with carbon monoxide (CO) and oxygen (O₂) in the presence of a catalyst system containing as a catalyst cupric methoxychloride Cu(OMe)Cl, and as an additive for the catalyst a phosphoramide of the formula ##STR11## where R₁ and R₂ are (C₁ -C₈) alkyl, alkylene or oxa-, thia-, aza-alkylene groups; and recovering the organic carbonate product.
 2. The method of claim 1, wherein the phosphoramide is hexamethylphosphoramide ##STR12##
 3. The method of claim 1, wherein the phosphoramide is hexaethylphosphoramide ##STR13##
 4. The method of claim 1, wherein the phosphoramide is tri(pentamethylene)phosphoramide ##STR14##
 5. The method of claim 1, wherein the phosphoramide is tri(oxapentamethylene)phophoramide ##STR15##
 6. The method of claim 1, wherein the method is carried out at a temperature of 20° C. to 170° C. and a pressure of 10 to 2000 psi for a period of 0.25 to 2.0 hours.
 7. The method of claim 6, wherein the method is carried out at a temperature of 80° C. to 120° C. and a pressure of 150 to 600 psi for a period of about 0.5 hours.
 8. The method of claim 6, wherein the method is carried out at a temperature of about 90° C. and a pressure of about 150 psi for a period of about 0.5 hour.
 9. The method of claim 1, wherein between about 0.01 and about 50 parts of catalyst per 100 parts of methanol are present. 